Remove load_balancing_atomic_replica feature flag

This has been enabled on GitLab.com for more than a month, and it
appears to have solved intermittent merge failures on protected
branches: https://gitlab.com/gitlab-org/gitlab/-/issues/247857.

Relates to feature flag rollout issue:
https://gitlab.com/gitlab-org/gitlab/-/issues/291193

Changelog: changed

config/feature_flags/development/load_balancing_atomic_replica.yml

----
-name: load_balancing_atomic_replica
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/49294
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/291193
-milestone: '13.11'
-type: development
-group: group::database
-default_enabled: true

lib/gitlab/checks/matching_merge_request.rb

@@ -28,11 +28,7 @@ module Gitlab
           # report no matching merge requests. To avoid this, we check
           # the write location to ensure the replica can make this query.
           track_session_metrics do
-            if ::Feature.enabled?(:load_balancing_atomic_replica, @project, default_enabled: :yaml)
-              ::Gitlab::Database::LoadBalancing::Sticking.select_valid_host(:project, @project.id)
-            else
-              ::Gitlab::Database::LoadBalancing::Sticking.unstick_or_continue_sticking(:project, @project.id)
-            end
+            ::Gitlab::Database::LoadBalancing::Sticking.select_valid_host(:project, @project.id)
           end
         end
 

spec/lib/gitlab/checks/matching_merge_request_spec.rb

@@ -56,6 +56,9 @@ RSpec.describe Gitlab::Checks::MatchingMergeRequest do
       before do
         expect(::Gitlab::Database::LoadBalancing).to receive(:enable?).at_least(:once).and_return(true)
         allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:all_caught_up?).and_return(all_caught_up)
+
+        expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:select_valid_host).with(:project, project.id).and_call_original
+        allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:select_caught_up_replicas).with(:project, project.id).and_return(all_caught_up)
       end
 
       shared_examples 'secondary that has caught up to a primary' do
@@ -84,39 +87,12 @@ RSpec.describe Gitlab::Checks::MatchingMergeRequest do
         end
       end
 
-      context 'with load_balancing_atomic_replica feature flag enabled' do
-        before do
-          stub_feature_flags(load_balancing_atomic_replica: true)
-
-          expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:select_valid_host).with(:project, project.id).and_call_original
-          allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:select_caught_up_replicas).with(:project, project.id).and_return(all_caught_up)
-        end
-
-        it_behaves_like 'secondary that has caught up to a primary'
+      it_behaves_like 'secondary that has caught up to a primary'
 
-        context 'on secondary behind primary' do
-          let(:all_caught_up) { false }
-
-          it_behaves_like 'secondary that is lagging primary'
-        end
-      end
+      context 'on secondary behind primary' do
+        let(:all_caught_up) { false }
 
-      context 'with load_balancing_atomic_replica feature flag disabled' do
-        before do
-          stub_feature_flags(load_balancing_atomic_replica: false)
-
-          expect(::Gitlab::Database::LoadBalancing::Sticking).not_to receive(:select_valid_host)
-          expect(::Gitlab::Database::LoadBalancing::Sticking).to receive(:unstick_or_continue_sticking).and_call_original
-          allow(::Gitlab::Database::LoadBalancing::Sticking).to receive(:all_caught_up?).and_return(all_caught_up)
-        end
-
-        it_behaves_like 'secondary that has caught up to a primary'
-
-        context 'on secondary behind primary' do
-          let(:all_caught_up) { false }
-
-          it_behaves_like 'secondary that is lagging primary'
-        end
+        it_behaves_like 'secondary that is lagging primary'
       end
     end
   end